Thermal shunt for trip units



June 30, 1964 G. J. GlEssNER 3,139,497

THERMAL sHuNT FOR TRIP UNITS Filed June 14, 1960 Y I N V EN TOR. @fa/e @E @5m/5e United Statesh Patent Omoo 3,139,497 Patented June so, 1964 3,139,497 THERMAL SHUNT FOR TREP UNITS George J. Giessner, Oaltlyn, NJ., assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed June 14, 1960, Ser. No. 35,995 11 Claims. (Cl. 20G- 116) This invention relates to molded case circuit breakers in general and more Vparticularly to the construction of the trip units and the manner in which the bimetal is heated.

Molded case circuit breakers are often provided with removable and replaceable trip units. In this event the bimetalelements are not iixedly mounted to the base of the circuit breaker proper but are instead mounted to the frame of the interchangeable trip unit. The interchangeable trip unit includes a latch means which is unlatched upon predetermined deilection of the bimetal andl upon unlatching permits the circuit breaker mechanism to operate to its open circuit position.

When the trip unit is changed or when the conductor receiving lug is screwed to the terminal strip there is the possibility of misalignment which is taken care of in prior art breakers by constructing the terminal strip as a flexible member. If the terminal strip is not constructed as a flexible member the strain upon the load terminal will cause a change in calibration of the time delay tripping device. This change in calibration is brought about by the fact that the bimetal is fastened directly to the terminal strip so that any distortion of the terminal strip will influence the position of the bimetal relative to the latch.

Thus, in prior art circuit breakers of this type the terminal strips were either made thin enough to be exible or were constructed of several laminations of thin material. If the terminal strip is made thin enough to be flexible this severely limits the current rating of the circuit breaker since heating of the terminal strip becomes excessive. The laminated construction has been found objectionable because of bulk and excessive expense of construction.

In the instant invention a solid terminal strip is utilized but the construction is such that any strain on the strip caused by an attempt to align otherwise misalignment parts will not be transmitted to the bimetal. This is accomplished by utilizing an indirectly heated bimetal which is connected to the terminal strip by means of a thermal shunt. The shunt is constructed of copper laminates and allows enough movement of the terminal strip to compensate for misalignment but still provides good heat transfer from the heater portion of the terminal strip to the bimetal. The bimetal is rigidly secured to the frame of the trip unitso that the thermal calibration will remain constant even though the terminal strip is severely strained during the alignment process.

The heater portion of the terminal strip is at and of reduced cross-sectional area whereas in the prior art devices the heater was comprised of a looped section which was extremely bulky. Thus, by utilizing the teachings of this invention not only is calibration unaffected by misalignment of the terminal strip and the circuit breaker base but a more compact construction for the circuit breaker is possible since the bimetal heat is compact.

Accordingly, a primary object of this invention is to provide a circuit breaker wherein mechanical strain on the terminal strip will not be transmitted to the bimetal to cause the thermal calibration of the circuit breaker to be upset.

Another object is to provide a novel means for indirectly heating the bimetal of an automatic trip unit whereby the overall size of the circuit breaker may be reduced. I

Still another object is to provide economical means for indirectly heating a bimetal in a circuit breaker of high current rating.

A further object is to provide a circuit breaker in which there is an indirect heated bimetal actuated by means of heat generated at a flat portion of the terminal strip.

A still further object is to provide a circuit breaker in which the bimetal is xedly secured to a frame and is connected to a heat generating means by a shunt of laminated construction.

These as well as other objects of the instant invention shall become readily apparent after reading the following description of the accompanying drawings in which:

FIGURE 1 is a cross-section of a three-phase circuit breaker incorporating the instant invention with the crosssection being taken through the center phase.

FIGURE 2 is a plan view of the interchangeable trip unit of the circuit breaker of FIGURE l.

FIGURE 3 is a cross-section of FIGURE 2 taken through line 3-3 of FIGURE 2.

Now referring to the figures, circuit breaker 10 includes a molded housing having a base 11 and a removable cover 12. Circuit breaker 10 is a three-phase unit having identical current paths for each phase which comprises terminal strip 13, stationary contact 14, movable contact 15, contact arm 16, tlexible braid 17, conducting brackets 18 and terminal strip 19. It is to be understood that terminal strips 13 and 19 will each be v y provided with suitable cable receiving lugs (not shown).

A single mechanism indicated generally as 20 is provided for moving cooperating contacts 14, 15 into and out of engagement either manually, by means of operating handle 21, or automatically in response to a fault current through the action of automatic trip unit 25 whose operation will be hereinafter explained.

Mechanism 20 is of a type well known to `the art, such as that illustrated in U.S. Patent 2,421,853, which acts through transverse tie bar 22 to bring about the simultaneous operation of all three phases.

Automatic trip unit 2S is an interchangeable device of the type well known to the art. That is, trip unit 25 may readily be removed and replaced by another trip unit having the same physical size but differing trip characteristics as will be hereinafter explained.

Automatic trip unit 25 includes a frame means comprising vertical partition 26 having secured to the forward side thereof metal bracket 27. Latch lever 28: is pivotally mounted at pin 29 to bracket 27. Latch lever 28 includes resetting pin 30 and horizontal latch surface 31 both positioned forwardly of partition 26. Latch lever 28 further includes arm 32 which extends through an aperture in partition 26 and passes the rear surface of partition 25.

The free end of arm 32 is engageable by latch member 33 carried by common tripper bar 34 in a manner Well known Ito the art. Common tripper bar 34 is pivotally mounted to bearings 35, 36 secured to the rear of partition 26. Common tripper bar 34 carries a downward extension 37 for each phase of circuit breaker 10. An adjusting screw 38 mounted to extension 37 is engageable by the free end of bimetal element 39 upon deflection Ithereof to the right with respect to FIGURE l.

When this deliection is suicient tripper bar 34 will have been rotated counterclockwise with respect to FIG- URE 2 against the force of a biasing spring (not shown) a distance suicient to cause latch member,l 33 to release arm 32 of latch lever 28. This in turn permits latch lever 28 to pivot in a clockwise direction about pin 29 being urged in this direction through the action of the main operating springs (not shown) of circuit breaker g at the lower end thereof with this intimate contact being brought about through the action of clamp member 43. The foot of shunt 44 is secured to terminal 19 at a portion intermediate the ends thereof by means of screw 45 which is received by a suitable threaded aperture in terminal strip 19.

The portion 46 of terminal strip 19 to whichrshunt 44 is secured is of a reduced cross-sectional area of terminal strip 19. It follows that the resistance of portion 46 is accordingly higher so that heating in region 46 is much n greater than heating in the remainder ofterminal strip 19. Heat generated at portion 46 is conducted through shunt 44 to bimetal 39 to cause heat thereof.

Each of the terminal strips 19 is removably secured to base 11 by means of two machine screws 49 which pass `through clearance holes in brackets 18 and clearance holes 51 in terminal strip 19 to be received by the threaded apertures of base inserts 52. In addition, machine screw 53 extends upwardly through base-11 to be received by threaded aperture 54 in terminal strip v 19. Machine screw 55 is received by threaded apertures in the terminal strip of the center phase after passing through a clearance hole (not shown) in bracket 27 thereby securing frame means 26, 27 in place. y

It is to be noted that even if the apertures S1, S4 are not precisely aligned with the cooperating apertures in base 11 so that shunt 44 must be bent during installation of trip unit 25, the mechanical strain will not be transmitted to bimetal 39 to cause repositioning thereof with respect to adjusting screw 38. Thus, even though heater shunt 44 is subjected to mechanical strain the calibration of trip unit 25 remains unchanged.

It is also to be noted that strip 19 isa solid and continuous member rather than a member which is laminated or a member whose central portion is cut away and replaced by a high resistance material. It is further to be noted that heater portion 46 is flat rather than being looped as ink conventional means for indirectly heating Vbimetallic trip elements. Thus, the construction of terminal strip 19 including the heater portion thereof is extremely economical.

Although I havehere described a preferred embodiment of my novel invention, many variations and modifications will now be apparent'to those skilled in the art, and I therefore prefer to be limited, not by the specific disclosure herein, but only by the appending claims.

I claim:

1. In a device of the class described a frame means, a terminal strip, a thermally responsive element xedly secured at a first end to said frame means at a point remote from said terminal strip, means preventing transmission of strain in said strip to said element, whereby lthe thermal calibration of said element is substantially independent of such stress; said means including a shunt connecting said element at said iirst end to said strip at a region intermediate the ends thereof, a latch means mounted to said frame means and operable to unlatching position by said element; a current path including said strip and exclusive of said shunt extending between the ends of said strip; said region having a reduced crosssectional Varea whereby upon passage of current through said path said region is at anelevated temperature with respect to the remainder of said strip.

2. In a device of the class described a frame means, a terminal strip, a thermally responsive element xedly secured at a first end to said frame means at a point remote it from said terminal strip; means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of such stress; said means including a shunt connecting said element at said first end to said strip at a region intermediate the ends thereof, a latch means mounted to said frame means and operable to unlatching position by said element; a current path including said strip and exclusive of said shunt extending between the ends of said strip; said region having a reduced cross-sectional area whereby upon passage of current through 'said path said region is v at an elevated temperature with respect to the remainder of said strip; said terminal strip comprising asolid member. t

3. In a device ofthe class described a frame means, a terminal strip, a thermally responsive element iixedly secured at a rstV end to said frame means at a point remote from said terminal strip; means preventing transmissionV of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of such stress; said means including a shunt connecting said element at said rst end to said strip at a'region intermediate the ends thereof, a latch means mounted to said frame means and operable to unlatching position by said element; a current path including said strip and exclusive of said shunt extending between the ends of said strip;

said region having a reduced cross-sectional area whereby upon passage of current through said path said region is at an elevated temperature with respect to the remainder of said strip; said terminal strip comprising a solid member with said region being substantially at.

4. In a device of the class described a frame means, a thermally responsive element xedly secured at a rst end to said frame means, a terminal strip; means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of such stress; said means including a shunt connectingsaid element at said rst end to said strip at a region intermediate the` ends thereof, a latch means mounted to said'frame means and operable to unlatching position by said element; said shunt comprising a laminated member providing a good heat conducting path to said element for heat generated at said region; said element andr said shunt being connected as non-current car- Y mounted to said frame means and operable to unlatching position by said element; said region having a reduced cross-sectional area whereby upon passage yof current through said strip said region is at an elevated temperalture with respect to the remainder of said strip; said terminal stripk comprising a solid member; said shunt com-` prising a laminated member providing a good heat conducting path to said element for heat generated'at said region; said element and said shunt being connected as non-current carrying members. v

6. A multiphase circuit breaker including for each phase thereof a thermally responsive element, a terminal strip, and means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said elementl is substantially independent of such stress; said means'including a shunt connectingsaid element at a first end thereof to said strip at a region intermediate the ends thereof; a common tripper bar engageable by..

second ends of said elements upon deilection thereof;

said bar carrying latch means which is uniatched upon deection of any of said elements to a predetermined position; a frame means to which said bar is mounted;

each of said elements being iixedly mounted at said first ends to said frame means at points remote from said terminal strips.

7. A multiphase circuit breaker including for each phase thereof a thermally responsive element, a terminal strip, and means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of such stress; said means including a shunt connecting said element at a first end thereof to said strip at a region intermediate the ends thereof; a common tripper bar engageable by second ends of said elements upon deflection thereof; said bar carrying latch means which is unlatched upon deflection of any of said elements to a predetermined position; a frame means to which said bar is mounted; each of said elements being xedly mounted at said iirst ends to said frame means at points remote from said terminal strip; each of said regions having a reduced cross-sec tional area whereby upon passage of current through said strips said regions are at elevated temperatures with respect to the remainders of the respective strips.

8. A multiphase circuit breaker including for each phase thereof a thermally responsive element, a terminal strip, and means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of such stress; -said means including a shunt connecting said element at a first end thereof to said strip at a region intermediate the ends thereof; a common tripper bar engageable by second ends of said elements upon deflection thereof; said bar carrying latch means which is unlatched upon deflection rof any of said elements to a predetermined position; a

frarne means to which said bar is mounted; each of said elements being xedly mounted at said first ends to said frame means at points remote from said terminal strips; each of said terminal strips comprising a solid member.

9. A multiphase circuit breaker including for each phase thereof a thermally responsive element, a terminal strip, and means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of such stress; said means including a shunt connecting said element at a first end thereof to said Istrip at a region intermediate the ends thereof which is of reduced cross-sectional area; a common tpper bar engageable by second ends of said elements upon deflection thereof; said bar carrying latch means which is unlatched upon deliection of any of said elements to a predetermined position; a frame means to which said bar is mounted; each of `said elements being xedly mounted at said first ends to said frame means; each of said terminal strips comprising a solid member With said region being substantially lat.

10. A multiphase circuit breaker including for each phase thereof a thermally responsive element, a terminal strip, and means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of suchstress; said means including a shunt connecting said element at a first end thereof to said strip at a region intermediate the ends thereof; a common tripper bar engageable by second end-s of said elements upon deflection thereof; said bar carrying latch means which is unlatched upon deflection of any of said elements to a predetermined position; a frame means to which said bar is mounted; said element being ixedly mounted at said first ends to said frame means; each of said shunts comprising a laminated member providing good conducting paths to said elements for heat generated at connected ones of said regions; said elements and said shunts being connected as non-current carrying members.

11. A multiphase circuit breaker including for each phase thereof a thermally responsive element, a terminal strip, and means preventing transmission of strain in said strip to said element, whereby the thermal calibration of said element is substantially independent of such stress; said means including a shunt connecting said element at a first end thereof to said strip at a region intermediate the ends thereof; a common tripper bar engageable by second ends of said elements upon deflection thereof; said bar carrying latch means which is unlatched upon deflection of any of said elements to a predetermined position; a frame means to which said bar is mounted; said element being lixedly mounted at said iii-st ends to said frame means each of said shunts comprising a laminated member providing good conducting paths to said elements for heat generated at connected ones of said regions; said elements and said shunts being connected as non-current carrying members; said shunts being L- shaped.

References Cited in the file of this patent UNITED STATES PATENTS 2,047,739 Lingal July 14, 1936 2,089,716 Smith Aug. 10, 1937 2,116,791 Jackson May 10, 1938 2,154,703 Sandin Apr. V18, 1939 2,175,859 Tusing Oct. 10, 1939 2,668,897 Beach et al. Feb. 9, 1954 2,854,555 Edmunds Sept. 30, 1958 2,922,008 Cellerini et al. Jan. 19, 1960 3,097,277 Wingard et al. July 9, 1963 

1. IN A DEVICE OF THE CLASS DESCRIBED A FRAME MEANS, A TERMINAL STRIP, A THERMALLY RESPONSIVE ELEMENT FIXEDLY SECURED AT A FIRST END TO SAID FRAME MEANS AT A POINT REMOTE FROM SAID TERMINAL STRIP, MEANS PREVENTING TRANSMISSION OF STRAIN IN SAID STRIP TO SAID ELEMENT, WHEREBY THE THERMAL CALIBRATION OF SAID ELEMENT IS SUBSTANTIALLY INDEPENDENT OF SUCH STRESS; SAID MEANS INCLUDING A SHUNT CONNECTING SAID ELEMENT AT SAID FIRST END TO SAID STRIP AT A REGION INTERMEDIATE THE ENDS THEREOF, A LATCH MEANS MOUNTED TO SAID FRAME MEANS AND OPERABLE TO UNLATCHING POSITION BY SAID ELEMENT; A CURRENT PATH INCLUDING SAID STRIP AND EXCLUSIVE OF SAID SHUNT EXTENDING BETWEEN THE ENDS OF SAID STRIP; SAID REGION HAVING A REDUCED CROSSSECTIONAL AREA WHEREBY UPON PASSAGE OF CURRENT THROUGH SAID PATH SAID REGION IS AT AN ELEVATED TEMPERATURE WITH RESPECT TO THE REMAINDER OF SAID STRIP. 